Method for producing a microalloyed steel, a microalloyed steel produced using the method, and a combined casting/rolling installation

The present application is a national stage application of PCT application PCT/EP2022/064188, filed May 25, 2022, which claims priority to the European patent application, EP21178473, Jun. 9, 2021, the contents of which are incorporated by this reference.

The invention relates to a process for producing a microalloyed steel according to claim, to a microalloyed steel according to claim, and to an integrated casting-rolling plant according to claim.

WO 2019/020492 A1 discloses a rolling stand having a stand cooler for cooling of a steel strip.

US 2016/151814 A1 discloses a plant and a process for hot rolling of a steel strip.

EP 2 398 929 A1 discloses a high-strength and thin cast strip product and a production process therefor.

“Microstructural Evolution and Strengthening Mechanism of X65 Pipeline Steel Processed by Ultra-fast Cooling”, published in the Journal of Northeastern University (Natural Science) vol. 40, no. 3, 1 Mar. 2019, pages 334-338, XP009531477, ISSN 1005-3026, discloses a process for producing X65 pipeline steel.

Moreover, WO 2020/126473 A1 discloses cooling of a metal strip in a rolling stand.

AT 512 399 B1 discloses a process for producing a microalloyed piping steel in an integrated casting-rolling plant.

It is an object of the invention to provide an improved process for producing a microalloyed steel in an integrated casting-rolling plant, an improved microalloyed steel and an improved integrated casting-rolling plant.

This object is achieved by a process according to claim, by a microalloyed steel, especially a microalloyed piping steel, according to claim, and by an integrated casting-rolling plant according to claim. Advantageous embodiments are specified in the dependent claims.

It has been recognized that an improved process for producing a microalloyed steel in an integrated casting-rolling plant can be provided in that the integrated casting-rolling plant has a continuous casting machine with a mold, a single- or multi-stand prerolling train, a finish-rolling train having a first stand group with at least one first finish-rolling stand and a second stand group having at least one stand cooler. A metallic melt is cast in the mold to give a partly solidified thin-slab strand.

In this application, strand-cast strands with a thickness of ≤150 mm are referred to as thin-slab strands. The partly solidified thin-slab strand is supported, deflected and cooled. The thin-slab strand is rolled in the prerolling train to give a prerolled strip. The first stand group of the finish-rolling train finish-rolls the prerolled strip to give the finish-rolled strip. Immediately after the finish rolling, the finish-rolled strip is fed to the second stand group and the finish-rolled strip is force-cooled in the second stand group with retention of a thickness of the finish-rolled strip in such a way that a cooling rate of a core of the finish-rolled strip in the second stand group is greater than 20° C./s and less than 200° C./s.

This configuration has the advantage that—preferably in continuous operation—the microalloyed steel can be produced in a simple manner. In particular, it is thus also possible, for example, with a metallic melt containing 10% less microalloy elements (for example titanium, niobium and/or vanadium), corresponding, for example, to an X60 to X120 steel according to standard API 5L/IS03183:2007, to produce a microalloyed steel that meets the mechanical demands for the steel qualities according to the standard cited. By the method, it is thus possible to produce the microalloyed steel in a particularly simple and inexpensive manner.

In continuous operation of the integrated casting-rolling plant, a continuously produced thin-slab strand is prerolled and finish-rolled in uncut form, and the microalloyed steel is cut to bundle length for the first time after passing through the cooling zone.

In a further embodiment, the second stand group has a second finish-rolling stand, wherein the second finish-rolling stand, in a preparation step prior to casting of the metallic melt, is converted to the stand cooler by removing at least one working roll of the second finish-rolling stand and inserting at least one cooling beam into the second finish-rolling stand. In this way, it is possible to convert the integrated casting-rolling plant in a particularly simple manner.

In a further embodiment, a third surface temperature with which the finish-rolled strip leaves the second stand group is ascertained. The forced cooling in the second stand group is controlled by open-loop/closed-loop control depending on the third surface temperature and a third target temperature in such a way that the third surface temperature corresponds essentially to the third target temperature. This third target temperature is less than a ferrite-perlite transformation temperature, preferably less than a bainite start temperature, especially less than a martensite start temperature. This configuration has the advantage that it is possible to produce a particularly inexpensive and mechanically high-quality microalloyed steel having a particularly low level of microalloy elements.

In a further embodiment, a second surface temperature with which the finish-rolled strip leaves the first stand group is ascertained. The second surface temperature is also taken into account in controlling the forced cooling of the finish-rolled strip in the second stand group. In this way, it is possible to particularly accurately adjust the cooling rate of the core of the finish-rolled strip by means of the forced cooling.

In a further embodiment, the cooling rate of the core of the finish-rolled strip is 20° C./s to 80° C./s, especially 45° C./s to 55° C./s. It is advantageous when the cooling is continuous. This ensures that a high-strength, for example bainitic and/or martensitic, microalloyed steel can be produced.

In a further embodiment, the core of the finish-rolled strip is transported with a first exit temperature of 830° C. to 950° C., especially of 880° C. to 920° C., into the second stand group of the finish-rolling train. On exit of the finish-rolled strip from the second stand group, the core of the finish-rolled strip has a second exit temperature of less than 700° C., especially 350° C. to 700° C., preferably of 400° C. to 460° C.

In a further embodiment, the core of the finish-rolled strip is cooled, preferably continuously, from the first exit temperature to the second exit temperature within a time interval of 2 seconds to 40 seconds. This makes it possible to avoid unwanted changes in microstructure as a result of the continuous cooling in the finish-rolled strip.

In a further embodiment, within a time interval of 1 second to 15 seconds after the finish-rolling of the finish-rolled strip in the first stand group, the finish-rolled strip enters the second stand group. As a result of the short time interval, the finish-rolled strip is cooled down from a particularly high first exit temperature. Moreover, unwanted cooling of the finish-rolled strip between the first stand group and the second stand group is kept particularly low.

In a further embodiment, the integrated casting-rolling plant has a cooling zone downstream of the finish-rolling train based on a conveying direction of the finish-rolled strip and a winding device downstream of the cooling zone. Forced cooling of the finish-rolled strip in the cooling zone is deactivated and the finish-rolled strip is transported through the cooling zone from the second stand group to the winding device. In this way, it is possible to dry off the finish-rolled strip in the cooling train, such that the finish-rolled strip is wound up dry to give a coil. Moreover, wear to the cooling train is reduced and hence maintenance work for the cooling zone is minimized.

In a further embodiment, a grain size of the prerolled strip on leaving the prerolling train is 10 μm to 30 μm. The grain size of the prerolled strip between the prerolling train and entry into the first stand group grows to 20 μm to 60 μm, or the grain size is maintained. The grain size of the finish-rolled strip on rolling in the first stand group is reduced to 2 μm to 20 μm. In particular, the microstructure has a “pancake structure” when the finish-rolled strip exits from the first stand group. The grain size can be determined in the cooled prerolled stripand/or cooled finish-rolled stripin a cross section at the normal angle to conveying direction, for example by light microscopy and, for example, according to ISO643 in a middle (both in terms of width and thickness) of the respective strip. On the basis of the grain size measured, the grain size of the prerolled strip between the prerolling train and the finish-rolling train and/or of the finish-rolled strip can be ascertained, for example, by means of a mathematical model. An illustrative mathematical model is known, for example, from ISIJ International, vol. 32 (1992), no. 12, pages 1329 to 1338, published under the title “A Mathematical Model to Predict the Mechanical Properties of Hot Rolled C—Mn and Microalloyed Steels”.

In a further embodiment, a thickness of the prerolled strip on entry into the first stand group is 40 mm to 62 mm, especially 45 mm. The first stand group reduces the thickness of the prerolled strip to 10 mm to 25 mm, especially 16 mm to 20 mm. This thickness is suitable in particular for production of pipes from the microalloyed steel.

In a further embodiment, the metallic melt for an X60 or an X70 steel has a chemical composition in percent by weight of C 0.025-0.05%; Si 0.1-0.3%; Mn 0.07-1.5%, Cr<0.15%; Mo<0.2%; Nb 0.02-0.08%; Ti<0.05%; V<0.08%; N<0.008%; balance: Fe and unavoidable impurities. By comparison with AT 512 399 B1, for example, the process lowers the limits on carbon, silicon and chromium. Molybdenum can be added in order to increase strength.

The metallic melt for X80 to X120 steels, especially for X90 to X120 steels, preferably has a chemical composition in percent by weight of C 0.025-0.09%; Si 0.1-0.3%; Mn 0.07-2.0%, Cr<0.5%; Mo<0.5%; Nb 0.02-0.08%; Ti<0.05%; V<0.08%; Ni<0.5%; Cu<0.4%; N<0.01%; balance: Fe and unavoidable impurities.

An improved and inexpensive microalloyed steel, especially microalloyed piping steel having a thickness of 10 mm to 25 mm, especially of 16 mm to 20 mm, can be produced by the process described above. The microalloyed steel for an X60 or an X70 steel preferably has a chemical composition in percent by weight of C 0.025-0.05%; Si 0.1-0.3%; Mn 0.07-1.5%, Cr<0.15%; Mo<0.2%; Nb 0.02-0.08%; Ti<0.05%; V<0.08%; N<0.008%; balance: Fe and unavoidable impurities. The microalloyed steel for X80 to X120 steels preferably has a chemical composition in percent by weight of C 0.025-0.09%; Si 0.1-0.3%; Mn 0.07-2.0%, Cr<0.5%; Mo<0.5%; Nb 0.02-0.08%; Ti<0.05%; V<0.08%; Ni<0.5%; Cu<0.4%; N<0.01%; balance: Fe and unavoidable impurities.

The microalloyed steel advantageously has at least one of the following precipitates at room temperature: Ti(C,N), Nb(C,N) V(C,N) TiC, TiN, Ti(C,N), (Nb, Ti)C, (Nb, Ti)N, (Nb, Ti)(C,N), NbC, NbN, VC, VN, V(C,N), (Nb,Ti,V)(C,N), (Nb,V)C, (Ti,V)C, (Nb,V)(C,N), (Ti,V)(C,N), (Nb,V)N, (Ti,V)N, (Nb,Ti,V)C, (Nb,Ti,V)N. A precipitate density of the precipitates is 10-101/m, where the precipitates have an average size of 1 nm to 15 nm. Preferably, the precipitate density and/or the average size can be determined by transmission electron microscopy (TEM), where a precipitate size for determination of the average size of the precipitates should preferably be determined transverse to a conveying direction of the finish-rolled strip and at right angles to a cross section of the finish-rolled strip.

It has been recognized that an improved integrated casting-rolling plant for production of a microalloyed steel can be provided in that the integrated casting-rolling plant has a continuous casting machine with a mold, a single- or multi-stand prerolling train and a finish-rolling train having at least a first stand group and a second stand group. A metallic melt is castable in the mold to give a partly solidified thin-slab strand, and the prerolling train is feedable with the thin-slab strand.

The prerolling train is designed to roll the fully solidified thin-slab strand to a prerolled strip, with the prerolled strip being feedable to the finish-rolling train. The first stand group is designed to finish-roll the prerolled strip to a finish-rolled strip. Based on a conveying direction of the finish-rolled strip, the second stand group is downstream of the first stand group and has at least one stand cooler. The second stand group is designed, with retention of a thickness of the finish-rolled strip, to force-cool the finish-rolled strip in such a way that a cooling rate of a core of the finish-rolled strip in the second stand group is greater than 20° C./s and less than 200° C./s. This makes it possible to use, in a simple manner, an integrated casting-rolling plant that works, for example, in continuous operation and typically produces conventional finished steel strips to produce finish-rolled strips with microalloyed steel, especially with microalloyed piping steel. This allows the integrated casting-rolling plant to be utilized flexibly in order to produce thin sheets having a thickness of 0.8 mm to 2.5 mm and to produce the finish-rolled strip from the microalloyed steel with the abovementioned thickness of 8 mm to 25 mm.

In a further embodiment, the integrated casting-rolling plant has a cooling zone downstream of the second stand group based on the conveying direction of the finish-rolled strip and a winding device downstream of the cooling zone. In the case of forced cooling of the finish-rolled strip in the second stand group, forced cooling of the finish-rolled strip in the cooling zone is deactivated. The cooling zone is designed exclusively to transport the finish-rolled strip to the winding device and preferably to dry the finish-rolled strip. This configuration has the advantage that the integrated casting-rolling plant can be operated in a particularly energy-efficient manner. In addition, the finish-rolled strip can be wound up dry, such that corrosion to the finish-rolled strip is avoided.

In a further embodiment, the integrated casting-rolling plant has a third temperature measurement device and a control unit, where the third temperature measurement device and the second stand group have a data connection to the control unit. The third temperature measurement device, based on the conveying direction of the finish-rolled strip, is preferably disposed between the second stand group and the cooling zone and is designed to ascertain a third surface temperature of the finish-rolled strip. The control unit is designed, on the basis of the ascertained third surface temperature of the finish-rolled strip and a predefined third target temperature, to control the forced cooling of the second stand group. This configuration has the advantage that a closed-loop control circuit can be provided in order to control the cooling of the finish-rolled strip in the second stand group.

shows a schematic diagram of an integrated casting-rolling plantin a first embodiment.

The integrated casting-rolling planthas, for example, a continuous casting machine, a prerolling train, a first to third separating device,,, an intermediate heater, preferably a descaler, a finish-rolling train, a cooling zone, a winding deviceand a control unit. In addition, the integrated casting-rolling plantmay have a first to third temperature measurement device,,, for example a pyrometer.

The continuous casting machineis designed by way of example as a bow-type continuous casting machine. The continuous casting machinehas a ladle, a distributorand a mold. In operation of the integrated casting-rolling plant, the distributoris filled with a metallic meltusing the ladle. The metallic meltcan be produced, for example, by means of a converter, for example in a Linz-Donawitz process. The metallic meltis, for example, a steel melt. The metallic meltflows from the distributorinto the mold. In the mold, the metallic meltis cast to a thin-slab strand. The partly solidified thin-slab strandis drawn out of the moldand deflected in an arc into a horizontal, while being supported and solidified, by the configuration of the continuous casting machineas a bow-type continuous casting machine. The thin-slab strandis conveyed away from the moldin conveying direction.

It is particularly advantageous here when the continuous casting machinecasts a continuous thin-slab strandand feeds it to a prerolling traindownstream in conveying direction of the thin-slab strand. In this embodiment, the prerolling trainfollows on directly from the continuous casting machine.

The prerolling trainmay have one or more prerolling standsarranged successively in the conveying direction of the thin-slab strand. The number of prerolling standsis choosable essentially freely and is dependent on a format of the thin-slab strandand on a desired thickness of the prerolled strip. In this embodiment, three prerolling standsare provided for the prerolling trainshown inby way of example. The prerolling trainis designed to roll the thin-slab strand, which is hot when fed into the prerolling train, to a prerolled strip.

The first and second separating devices,are downstream of the prerolling trainbased on the conveying direction of the prerolled strip. The second separating deviceis spaced apart from the prerolling train, based on the conveying direction of the prerolled strip. It is possible for an outward conveying device to be disposed between the first separating deviceand the second separating device. It is also possible to dispense with the second separating device. The first and/or second separating devices,may be designed, for example, as drum shears or pendulum shears.

In the production of the microalloyed steel/microalloyed piping steel, the integrated casting-rolling plant can be operated in continuous operation, i.e. in that the thin-slab strand enters the prerolling trainin uncut form, the prerolled strip passes through the first and/or second separating devices in uncut form, and the prerolled strip in uncut form is finish-rolled in the finish-rolling train, and only after passing through the cooling zoneis cut to bundle length.

Based on the conveying direction of the prerolled strip, in this embodiment, the second separating deviceis followed by the intermediate heaterby way of example. The intermediate heateris designed, for example, as an induction furnace. A different configuration of the intermediate heaterwould also be conceivable. The intermediate heateris upstream of the finish-rolling trainand the descalerbased on the conveying direction of the prerolled strip. The descaleris directly upstream of the finish-rolling trainand downstream of the intermediate heater.

The finish-rolling train, in this embodiment, has a first stand groupand a second stand group. The first stand groupis upstream of the second stand groupbased on the conveying direction of the prerolled strip. The first stand groupmay have, for example, two to four first finish-rolling stands. The first finish-rolling standsare arranged in series based on the conveying direction of the prerolled strip. The first stand groupfollows on directly from the descaler, if the descaleris provided, based on the conveying direction of the prerolled strip. If the descaleris dispensed with, the first stand groupfollows directly on from the intermediate heater.

The second stand grouphas at least one, preferably two, second finish-rolling stand (s), where the first finish-rolling standand the second finish-rolling standmay be of identical construction. In this embodiment, however, it is at least the case that, in addition, the second finish-rolling standhas a means of conversion to a stand cooler. In this embodiment, the two second finish-rolling standshave each been converted to one stand cooler. In the function of the stand cooler, the second finish-rolling standno longer performs a rolling process.

In addition, the second stand groupmay have at least one intermediate cooler. The intermediate coolermay be disposed in each case between two finish-rolling stands,. In this embodiment, the second stand grouphas, by way of example, two intermediate coolers, where a first of the two intermediate coolersis disposed by way of example between the last first finish-rolling standof the first stand groupin conveying direction and the foremost second finish-rolling standin conveying direction. It is also possible for a further intermediate coolerto be disposed between the two second finish-rolling stands. It is also possible to dispense with the intermediate coolersor to provide just one of the two intermediate coolers.

As already elucidated above, in this embodiment, the second finish-rolling standhas been converted to the stand cooler. The means of conversion may be implemented in that the second finish-rolling standhas a changeover device (not shown). In one configuration of the second finish-rolling standas second rolling stand, the changeover device secures at least one insert and an upper and/or lower working roll,(shown by dashes in) in the second finish-rolling stand. In the configuration as second rolling stand with at least the upper and/or lower working roll,, the second finish-rolling standis designed to roll the prerolled strip.

In the configuration of the second finish-rolling standas stand cooler, the changeover device secures means of cooling a finish-rolled striprather than the insert and the lower and/or upper working rolls,. The insert and the upper and/or lower working rolls,have been withdrawn. The configuration of the second finish-rolling standas stand coolerand the envisaged means of cooling the finish-rolled stripare discussed hereinafter. The changeover device allows the second finish-rolling standto be converted rapidly and easily between the second rolling stand for rolling the prerolled stripand the stand cooler.

The stand coolerand the intermediate coolereach have at least one cooling beam as means of cooling. The cooling beams of the stand coolerand/or of the intermediate coolerare preferably respectively disposed both on the top side and on the bottom side relative to the finish-rolled strip, in order to particularly rapidly and effectively cool the finish-rolled stripon both sides. In the stand cooler, the cooling beam is secured by means of the changeover device in place of the upper and/or lower working roll,.

It is possible by virtue of the configuration shown into provide a total of 16 cooling beams, for example, by means of two intermediate coolersand two stand coolers. It is possible here, for example, for each stand coolerto have two cooling beams disposed on the top side and two cooling beams disposed on the bottom side relative to the finish-rolled strip. It is pointed out that this configuration is an illustrative configuration of the second stand group. It will be appreciated that it would also be conceivable to design the second stand groupdifferently. For example, it is possible to dispense with at least one of the intermediate coolers. A different arrangement of the intermediate cooler (s)would also be conceivable. The arrangement and/or number of cooling beams is also illustrative. For instance, the number of cooling beams, in one development, may be increased or reduced. It is also conceivable that the cooling beams are disposed only on the top side or bottom side of the finish-rolled strip.

In this embodiment, the upper and/or lower working rolls,are detached in order to provide sufficient build space for the cooling beams in the second finish-rolling standthat has been converted to the stand cooler. In one development, it would also be possible for just the upper or lower working roll,to be removed.

In operation of the integrated casting-rolling plant, the first finish-rolling standsfinish-roll the prerolled stripfed into the first stand groupto the finish-rolled strip. The cooling zoneis downstream of the finish-rolling trainbased on a conveying direction of the finish-rolled strip. In conveying direction of the finish-rolled strip, the third separating deviceis downstream of the cooling zone. The third separating deviceis disposed here between the winding deviceand the cooling zone. The third separating devicemay be designed, for example, as drum shears or pendulum shears.

The control unitcomprises a control device, a data storage mediumand an interface. The data storage mediumhas a data connection to the control deviceby means of a first data connection. The interfacelikewise has a data connection to the control deviceby means of a second data connection.